A titanium (Ti) alloy has been used as high strength parts in the fields of military apparatuses, space developments, aircraft and racing cars, owing to its high specific strength and high specific toughness. However, it has been considered that the Ti alloy is difficult to be used for mass-produced parts for reasons such as high cost of raw materials, difficulty in melting and casting, and poor in yield.
Recently, a sintered Ti alloy material has been developed that solves the high cost and low productivity of the Ti alloy, and realizes high strength and high fatigue strength. For example, a Ti-based composite material composed of a matrix of an .alpha.-type, .alpha.+.beta.-type or .beta.-type Ti alloy and from 5 to 50% by volume of a TiB solid solution, and its production process are reported in Japanese Unexamined Patent Publication No. 5-5142. In this process, a TiB solid solution, which is inherently difficult to react with a Ti alloy, is selected as reinforcing particles, so as to improve the strength, rigidity, fatigue strength, wear resistance and heat resistance.
As a production process applicable to mass-produced parts, in which a TiB solid solution is uniformly dispersed, a process is proposed in which a packing density of Ti powder is increased to a prescribed value by controlling the shape of the Ti powder, and miniaturization of residual pores is thus achieved. There is described that according to these procedures, even if Ti powder of low cost is employed, impurities and foreign matters, which should deteriorate the characteristics, are positively used as an agent for improving the characteristics, so as to obtain a sintered Ti alloy material having excellent mechanical properties. According to this process, while the detailed mechanism is not clear, a dense sintered body having a density of 99% can be obtained when 1.8% by weight of B is added, and that having a density of 96% can be obtained even when 3.6% by weight of B is added. Thus, a sintered body having excellent strength, rigidity and fatigue strength can be obtained.
As a process for highly densifying a sintered body, a Ti alloy for powder sintering of high density, in which the amounts of Fe, Mo, Al, V and O are limited and the balance is composed of Ti and unavoidable impurities, is proposed in Japanese Unexamined Patent Publication No. 5-171321. In this process, Fe having a small diffusion rate in a Ti alloy and Mo having a large diffusion rate in a Ti alloy are combined, and sintering is thus conducted within a short period of time at a low temperature to obtain a high density. Because the addition of only Fe is liable to bring about formation of pores in an alloy component due to the Kirkendall effect, Mo having a small diffusion rate is combined to suppress that phenomenon. By further adding suitable amounts of Al and V and controlling the content of O, it provides to develop a sintered Ti alloy having a desired strength.
In Japanese Unexamined Patent Publication No. 5-5142, the excellent strength, rigidity and fatigue strength that have not been obtained by the conventional Ti alloy can be obtained up to the addition amount of B of 1.8% by weight. However, the density of the sintered body is lowered, and along with this, the level of strength is also lowered when B is added in an amount more than 1.8% by weight.
In Japanese Unexamined Patent Publication No. 5-171321, a production process is proposed in which the addition amounts of Fe and Mo are optimized to accelerate the densification at a low temperature within a short period of time, so that a sintered Ti alloy having desired strength is produced at low cost. However, this process still cannot overcome the low rigidity, i.e., the defect of a Ti alloy. Therefore, it cannot be applied to parts requiring rigidity, such as parts for automobiles.
Japanese Examined Patent Publication No. 1-29864 proposes a process in which a hot isostatic pressing is conducted after sintering to densify a sintered Ti alloy. However, a great increase in production cost cannot be avoided on production of mass-produced parts according to this process. Thus, it cannot be applied to inexpensive mass-produced parts, such as parts for automobiles.
As another process for densifying, an activation sintering method utilizing a liquid phase of a low melting point, such as a liquid phase of Ti--Fe, Ti--Ni and Ti--Co, in sintering can be exemplified. However, the process utilizing a liquid phase has a drawback that under the conditions where the densification can be conducted, the phase obtained by solidifying the liquid phase is generally brittle, and sufficient strength characteristics cannot be obtained. Furthermore, there is a case in which a large amount of the liquid phase due to the composition segregation becomes pores as a result of effusion, and a sintered body having a high density cannot be obtained.
As a result of earnest study and various systematic experimentations conducted by the inventors to solve the problems associated with the conventional processes, the present invention has been accomplished.